This invention relates to an encoder of a multi-pulse type for use in encoding a speech signal into a plurality of excitation pulses.
A conventional encoder of the type described is disclosed in U.S. Pat. No. 4,716,592 issued to Kazunori Ozawa et al and assigned to the instant assignee.
In the encoder, the speech signal is divided into a sequence of frames or segments. The speech signal is encoded into a plurality of excitation pulses by the use of a pulse calculating method. The encoder therefore comprises a pulse calculating unit for carrying out the pulse calculating method. The pulse calculating unit calculates a predetermined number of excitation pulses for each frame. Each of the excitation pulses has an amplitude and a location determined by the speech signal. Excitation pulse information representing the amplitude and the location is transmitted from the encoder to a decoder which is used as a counterpart of the encoder. The decoder decodes the excitation pulse information into a decoded signal and outputs the decoded signal as a synthesized speech signal.
Generally speaking, when the speech signal has a low pitch frequency, the speech signal can be characterized by a small number of excitation pulses. The decoder can therefore produce a favorable synthesized speech signal regardless of the number of the excitation pulses. On the other hand, when the speech signal has a high pitch frequency, the speech signal must be characterized by a large number of excitation pulses.
In the encoder, the predetermined number of the excitation pulses has, however, been constant regardless of the pitch frequency of the speech signal. This means that the synthesized speech signal may have a degraded quality when the speech signal has a high pitch frequency.
In the meanwhile, another encoder is disclosed in U.S. Pat. Application Ser. No. 153,290 filed Feb. 4, 1988, by Taguchi and assigned to the instant assignee. Briefly, the encoder comprises a detecting unit for detecting electric power level in response to the speech signal to produce a detection signal. The encoder further comprises a processing unit for processing an excitation pulse sequence to produce a processed signal in accordance with the detection signal. The processing unit comprises an extractor for extracting a plurality of excitation pulses in accordance with the detection signal to produce extracted excitation pulses as the processed signal. In the encoder, the number of the excitation pulses is variable. However, waste calculation for the excitation pulses is carried out because several excitation pulses are thinned out in the extractor.